A number of coupling techniques have been developed to efficiently couple power between optical fibers and optical devices, especially between a semiconductor laser and an optical fiber, in an optical communication system. For example, an increase in the coupling efficiency and, thus, a decrease in the coupling loss between the laser and the fiber, permits an increase in the repeater spacing in long-haul transmission systems. However, efficient coupling of semiconductor lasers to optical fibers has been a problem of general concern since the advent of optical fiber communications. In a single mode fiber implementation, the laser light power that can be launched into the single mode fiber suffers a loss on the order of 7–11 dB using conventional butt-joint coupling. Thus, coupling efficiency has been universally improved by using either a microlens on the end of the fiber to match the modes of the laser and optical fiber or by bulk optics or by a combination of microlenses and bulk optics. Microlenses have been more commonly used because of their ease of fabrication and packaging.
Generally, as a result of coupling inefficiencies, a percentage of the laser output is not utilized. Thus, the laser has to be run at a correspondingly higher current to yield the same coupled power into fiber that a more efficient coupling scheme could provide. In addition, operation of the laser at higher currents results in greater heat to be dissipated and raises questions of long term stability and reliability of the laser itself. Thus, a number of variations have been proposed or suggested to the shape or the manner in which the microlenses are fabricated, in order to gain improvements in the coupling efficiency of such microlenses.
For example, U.S. Pat. No. 5,011,254, issued to C. A. Edwards and H. M. Presby, assigned to the assignee of the present invention and incorporated by reference herein, discloses the use of hyperbolically-shaped microlensed fibers that provide greater than 90 percent coupling efficiencies for coupling light from single-mode light sources, such as semiconductor lasers with symmetric modal outputs, to single-mode fibers and from the single-mode fibers to detectors. While such microlenses afford relatively high coupling efficiency, they are useful only for single-mode lasers having a symmetric modal output, where the output beam profiles are generally circular, and therefore have ellipticity ratios that are close to 1:1.
Many lasers, however, have a highly elliptical beam shape emanating from the laser facet and exhibit strong modal asymmetries. Thus, in lasers exhibiting such strong modal asymmetries, the use of symmetric microlenses for coupling such lasers to fibers leads to a significant decrease in the coupling efficiencies. U.S. Pat. No. 5,256,851, issued to H. M. Presby, assigned to the assignee of the present invention and incorporated by reference herein, discloses methods and apparatus for fabricating asymmetric microlenses on single-mode optical fibers.
While the above-described coupling techniques are satisfactory for single-mode optical environments, there are increasing applications of multimode lasers and multimode fibers in optical communication systems. Generally, multimode lasers exhibit a non-symmetrical, highly elliptical beam shape emanating from the laser facet. The ellipticity ratios for such multimode lasers are generally on the order of 3:1. Thus, a need therefore exists for a method and apparatus for efficiently coupling using a microlens. A further need exists for a microlens that accepts an optical beam having a highly elliptical beam shape and converts the optical beam for acceptance by the circular multimode optical fiber. Unlike the single mode asymmetric situation, where the lens is formed only over the core region, which may encompass, for example, 10 microns out of the 125 micron diameter, in the multimode case, the core can be a substantial portion of the overall diameter, such as 100 microns out of the 125 micron diameter. Thus, the lens must have a much larger area, requiring the development of new fabrication methods.
U.S. patent application Ser. No. 09/244,635, filed Feb. 4, 1999, entitled “Apparatus, System and Method for Coupling Multimode Light Sources to Multimode Optical Fiber,” assigned to the assignee of the present invention and incorporated by reference herein, discloses a hyperbolic lens formed on a multimode fiber to facilitate the optical coupling of a multimode laser to a multimode fiber. While the disclosed hyperbolic lens provides significant improvements for efficiently coupling a multimode laser to a multimode fiber, the disclosed hyperbolic lens suffers from a number of limitations which, if overcome, could further expand the utility and efficiency of such multimode microlenses. Specifically, the disclosed hyperbolic lens requires the tailored shaping of the end of the fiber to achieve the hyperbolic shape, thereby increasing the fabrication costs due to the required tolerances that must be achieved.